For the measurement of gas concentrations and especially the concentrations of carbon monoxide (CO), nitrogen oxides (NO) and hydrocarbons (CxHy), semiconductor sensors are used in the automotive field because of their low cost. Most of the semiconductor sensors are conductivity-based SnO2 sensors. The measurement results can serve, for example, to open or close air circulation flaps in an automobile.
The above-mentioned sensors are characterized, apart from their low cost, by a good sensitivity to the gases to be measured. On the drawback side, however, these sensors have a number of side effects which complicate the signal evaluation. Reducing gases, like for example carbon monoxide contribute to an increase in the conductivity of the semiconductor sensors. Oxidizing gases like for example nitrogen oxide contribute to a reduction in the conductivity of the semiconductor sensors. In addition, the strong adsorption of water on the surface of the SnO2 semiconductor sensor gives rise to a detrimental side effect. The bound water increases the conductivity of the gas-sensitive SnO2 layer significantly. The amount of water adsorbed on the sensitive SnO2 layer is dependent to a high degree upon the temperature. As a result the change in the conductivity of the SnO2-layer is strongly temperature dependent. At a temperature below 200° C. substantially greater quantities of water can be bound to the semiconductor sensors than at higher temperatures. The adsorbed water quantities can be determined by means of a TDS [Total Dissolved Solids] measurement. After a certain time, a temperature-dependent equilibrium develops between adsorbed and desorbed water. Upon a change in temperature, the time constant to reach a new equilibrium is between several minutes and several hours. The time constant depends upon prevalent environmental conditions.
This effect arises especially in the phase following the switching on of the semiconductor sensor or in operation and is especially detrimental in its manifestations.
If the sensor is stored at ambient temperature for a period of several weeks, in the course of this period there will be an equilibrium for this temperature at saturation between adsorbed water and desorbed water. This equilibrium is referred to hereinafter also as the saturation equilibrium. To be able to carry out gas measurements with the sensor, the sensor is brought to an operating temperature of about 330° C. The increased temperature of 330° C. by contrast with the storage temperature means that water will be desorbed until a new saturation equilibrium is formed. During this period of time, as a consequence, the conductivity will drop continuously even if the gas concentration should remain constant. The resulting drop in conductivity is correlated with a conductivity change of the type which can result from a large increase in the NO concentration.
The result is that the measurement of the NO concentration during the interval in which a new saturation equilibrium is created is associated with significant errors in measurement.